CVE-2026-23519 — RustCrypto cmov: thumbv6m-none-eabi compiler emits non-constant time assembly when using cmovnz

Affected: RustCrypto utils

RustCrypto CMOV provides conditional move CPU intrinsics which are guaranteed on major platforms to execute in constant-time and not be rewritten as branches by the compiler. Prior to 0.4.4, the thumbv6m-none-eabi (Cortex M0, M0+ and M1) compiler emits non-constant time assembly when using cmovnz (portable version). This vulnerability is fixed in 0.4.4.

GHSA-2gqc-6j2q-83qp — RustCrypto Utilities cmov: `thumbv6m-none-eabi` compiler emits non-constant time assembly when using `cmovnz`

Affected: crates.io: cmov

### Summary

`thumbv6m-none-eabi` (Cortex M0, M0+ and M1) compiler emits non-constant time assembly when using `cmovnz` (portable version). I did not found any other target with the same behaviour but I did not go through all targets supported by Rust.

### Details

It seems that, [during `mask` computation](https://github.com/RustCrypto/utils/blob/9e555db060c80f4669d804f448a524a37d201b32/cmov/src/portable.rs#L78), an LLVM optimisation pass is detecting that [`bitnz`](https://github.com/RustCrypto/utils/blob/9e555db060c80f4669d804f448a524a37d201b32/cmov/src/portable.rs#L13) is returning 0 or 1, that can be interpreted as a boolean. This intermediate value is not masked by a call to `black_box` and thus the subsequent [`.wrapping_sub(1)`](https://github.com/RustCrypto/utils/blob/9e555db060c80f4669d804f448a524a37d201b32/cmov/src/portable.rs#L78C1-L78C84) can be interpreted as a conditional bitwise conditional not.

### PoC

This is an attempt at having a minimal faulty code. In a library crate with an up-to-date `cmov` as only dependency, the content of `src/lib.rs` is:

```rust #![no_std] use cmov::Cmov;

#[inline(never)] pub fn test_ct_cmov(a: &mut u8, b: u8, c: u8) { a.cmovnz(&b, c); } ```

The resulting assembly emitted (shown using `cargo asm --release --target thumbv6m-none-eabi` that uses [`cargo-show-asm`](https://crates.io/crates/cargo-show-asm)):

<details> <summary>Collapsed assembly</summary>

```asm .section .text.not_ct::test_ct_cmov,"ax",%progbits .globl…

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CVE-2026-23519 — RustCrypto cmov: thumbv6m-none-eabi compiler emits non-constant time assembly when using cmovnz

GHSA-2gqc-6j2q-83qp — RustCrypto Utilities cmov: `thumbv6m-none-eabi` compiler emits non-constant time assembly when using `cmovnz`